Photographic element and process

ABSTRACT

PHOTOGRAPHIC ELEMENTS INCORPORATING AN ELECTRICALLY INSULATING SUPPORT HAVING COATED THEREON AT LEAST ONE RADIATION-SENSITIVE SILVER SALT LAYER CAN BE PREPARED WITH LITTLE OR NO HYDROPHILIC COLLOID BINDER BY THE ELECTOSTATIC DEPOSITION OF DISPERSED PREFORMED, RADIATION-SENSITIVE SILVER SALT CRYSTALS FROM A FLUID MEDIUM, WHICH ELECTROSTATIC COATING PROCESS CAN ALSO BE USED TO DEPOSIT SUCH SILVER SALT CRYSTALS INCLUDING SILVER-DYE COMPLEX CRYSTALS WHICH HAVE BEEN PREVIOUSLY DISPERSED IN A HYDROPHILLIC COLLOID BINDER.

United States Patent Office Patented Sept. 12, 1972 3,690,918 PHOTOGRAPHIC ELEMENT AND PROCESS Warren J. Miller, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, NY. No Drawing. Filed Mar. 19, 1970, Ser. No. 21,174 lint. Cl. B44d 1/02; G03c 1/00 U.S. Cl. 117-34 12 Claims ABSTRACT OF THE DISCLOSURE have been previously dispersed in a hydrophilic colloid binder.

This invention relates to photography and more particularly to photographic elements having, as the lightsensitive component, a substantially binder-free layer of silver halide crystals and to a process for preparing both such elements and additional elements utilizing a hydrophilic colloid binder.

Conventional photographic elements employ a support material coated with a photographic emulsion that typically includes photosensitive silver halide crystals dispersed in a hydrophilic colloid such as gelatin. In many instances, gelatin or another hydrophilic colloid is a desirable, and necessary constituent of a photographic element. The use of a hydrophilic colloid, however, such as for example a binder to promote adequate adhesion between the photosensitive species and the support, can be attended by certain related disadvantages. Typical binder colloids absorb light of short wavelengths (e.g. ultraviolet rays); as such, the binder material can determine the short wavelength limit of usefulness for any given photographic element. Such a condition operates to impede preparation of high quality, ultraviolet-sensitive photographic elements such as those used in astronomical studies and as electron-recording elements. Moreover, developing agents are required to penetrate the colloid in order to develop a photographic image on the photosensitive material. Additionally, the hydrophilic colloid tends to attract moisture which can adversely affect the stability and image-producing capability of the photographic element, and special measures must be taken to ensure adequate adhesion between the normally hydrophobic support and the hydrophilic colloid binder. To avoid the use of colloid binder, it is known to prepare photographic elements having thin, binder-free layers of microcrystalline silver halide coated by vacuum deposition techniques wherein the silver halide crystals are formed in situ on the support. Such coating means, however, requires specialized apparatus to carry out the coating operation in a sealed system under conditions of elevated temperature and reduced pressure. Additionally, the binderless, microcrystalline silver halide layers so deposited are subject both to non-uniform photographic response and to deterioration upon storage, the deterioration generally resulting from such factors as pressure-caused abrasion fog. Additionally, presently known binderless silver halide elements are amenable to spectral sensitization only subsequent to the coating operation.

Accordingly, it is an object of this invention to provide new, substantially binder-free silver salt photographic elements.

Another object of this invention is to provide novel, substantially binder-free photographic elements employing preformed, presensitized silver salt crystals.

Another object of the present invention is to provide a novel electrostatic coating process for preparing both substantially binder-free silver salt photographic elements and photographic elements incorporating a hydrophilic colloid.

Additional objects and advantages will become apparent from a reading of the following specification and appended claims.

The objects of the present invention are accomplished both with photographic elements having an electrically insulating support coated with a substantially binder-free, radiation-sensitive layer of preformed silver salt crystals and with an electrostatic coating process for preparing both the subject elements and elements containing a hydrophilic colloid, which process includes:

(a) Dispersing radiation-sensitive silver salt crystals in a dielectric fluid medium,

(b) Contacting such a silver salt dispersion with an electrically insulating member bearing on a surface thereof an electrostatic charge of a polarity opposite to that of the dispersed silver salt, whereupon, dispersed silver salt is electrostatically attracted to and deposited upon said electrically insulating member to prepare a photographic element of the subject invention.

The term radiation-sensitive, as utilized herein, is descriptive of a chemical species (silver salts) that are activated by exposure to electromagnetic radiation to typically provide latent images that can be intensified by various photographic development techniques to provide visible images. The subject silver salt crystals are radiation-sensitive species that are light-sensitive. However, radiationsensitive comprehends responsiveness to activating radiation both within and without the visible portion of the spectrum. In addition to photosensitivity or visible light sensitivity, radiation-sensitive refers to the responsiveness of an activatable chemical species to a wide segment of electromagnetic radiation including, for example, X-radiation, ultraviolet rays, infrared radiation and the like.

The preformed, radiation-sensitive silver salt crystals used in the practice of this invention include those of the silver halides typically employed in gelatino-silver halide photographic emulsions, such as silver bromide, silver chloride and silver iodide. Additionally, mixtures of these halides are advantageously utilized, as are silver halide co-crystals such as, for example, silver bromoiodide, silver chlorobromide, silver chloroiodide, and the like silver halides.

The silver halides that function advantageously in the practice of this invention include sensitized preformed silver halide crystals that have been spectrally sensitized to additional portions of the spectrum, conventionally with sensitizing dyes such as cyanine dyes, carbocyanine and the like polymethine cyanine dyes, merocyanine dyes, styryl dyes, oxonol dyes, as well as additional sensitizers like substituted polycyclic quinones, pyrylium compounds and the like. Still additional spectral sensitizers are well known in the art. Spectrally sensitized silver halides can be dispersed in combination to obtain a composite dispersion containing silver salts sensitized to record various portions of the spectrum via the use of sensitizing dyes. As well as presensitized silver salt crystals, sensitization can be effected subsequent to preparing a photographic element, such as by bathing the element in a solution of sensitizing dye.

Additionally, the silver salt crystals useful herein include silver salts of sensitizing dyes, they generally being the complex reaction product of a water soluble silver salt and a spectral sensitizing dye. Particularly advantageous silver salts include those of the more highly solubilizing anions, and especially silver nitrate. The complexes are sensitive in general to the absorption spectra of the complexed sensitizing dye. The resultant preformed silverdye complex crystals are desirably dispersed in a hydrophilic colloid material prior to being coated on a support material. Useful spectral sensitizing dyes include, for example, dyes of the types commonly used to spectrally sensitize photographic silver halides and include such organic dyes as cyanine dyes, merocyanine dyes, oxonol dyes, hemicyanine dyes, styryl dyes, hemioxonol dyes, benzylidene dyes and the like. Such spectral sensitizers enter into complexes of varying strength with soluble silver salts. Particularly useful complex-forming dyes include those of the types noted above which also exhibit functional groups like C=S, CSH, C-SO H, CSO Na, HO=CH, CCN and CCOOH, such as the cyanine dye anhydro-3-ethyl-9-methyl-3'-(3- sulfobutyl)thiocarbocyanine hydroxide.

Preferred spectral sensitizing dyes used to prepare silver-dye complexes are merocyanine dyes such as:

(a) 3-carboxymethyl-5-[ (3-methyl-Z-thiazolidinylidene) ethylidene1rhodanine 420-560 m (b) (3-methyl-Z-thiazolidinylidene) ethylidene] rhodanine 460-570 mg,

(c) 5-[ (3-methyl-2-thiazolidinylidene) ethylidene] -2- thio-2,4-oxazolidinedione 400-560 ma,

((1) 3-ethyl-5- 3-methyl-Z-thiazolidinylidene)ethylidene]-2-thio-2,4-oxazolidinedione 430-540 mg,

(e) 1-methyl-5- 3-methyl-2-thiazolidinylidene ethylidene]-2-thiobarbituric acid 430-530 mg,

(f 3-carboxymetl1yl-5- (3-ethyl-2-benzoxazolinylidene ethylidene]rhodanine 420-580 mg,

(g) 5-(3-ethyl-2-benzoxazolinylidene)rhodanine 370-460 (h) 5-[(3-ethyl-2-benzoxazolinylidene)ethylidene] rhodanine 520-560 mg,

(i) 3-ethyl-5-[(3-ethyl-2-benzoxazolinylidene)ethylidene]-1-phenyl-2-thiohydantoin 520-560 mg,

(j) l-carboxymethyl-S- (3-ethyl-2-benzoxazolinylidene) ethylidene]-3-phenyl-2-thiohydantoin 520-560 me.

Other advantageous spectral sensitizing dyes include such dyes as those disclosed in French Pat. 1,453,635. Additional advantageous silver-dye complexes are described in US. Pat. 3,446,619.

These silver-dye complexes are conveniently prepared by mixing the dye with a water soluble silver salt. The resulting complex can be used in the reaction solution or colloidal suspension without being isolated. Theoretically, an equi-molar quantity of silver to dye is necessary to form the complex. However, it may be desirable to use an excess of silver depending upon the particular dye used and the concentration of the dye in solution. Dyes used in preparing the light-sensitive silver dye complexes are advantageously water soluble, but in the event that they are substantially insoluble in water, solutions can be obtained by dissolving them in sufficient water miscible solvent. The water miscible solvent is not critical but may be chosen from those which are compatible with any colloid which is used as a binder or coating medium. In the event that an organic solvent soluble silver salt is used, the reaction to form the complex can be carried out in a suitable solvent.

In addition to the unsensitized and sensitized silver salt crystals as noted hereinabove, such salts can, as noted above regarding silver-dye complexes, be dispersed first in a hydrophilic colloid material such as gelatin after which this existing dispersion is itself dispersed and coated according to the subject invention as long as each dispersed member, including the hydrophilic colloid, is substantially insoluble in the dispersion medium. In this fashion, a mixed grain color photographic element, such as those described in US. Pats. 2,388,859 and 2,614,925,

can be prepared to record full color positive images subsequent to photographic development. Where substantially complete removal of hydrophilic colloid is desired for a binderless coating, such removal is effectively accomplished by enzyme hydrolysis as described in Weiss, Ericson and Herz, J. Colloid and Interface Science, 23, p. 277 (1967).

In preparing the subject photographic elements, crystals of a radiation-sensitive silver salt or mixtures thereof are dispersed in a dielectric fluid medium by stirring or otherwise agitating the silver salt particles to prevent settling and maintain the particulate dispersion. A convenient agitation means for liquid media is provided by ultrasonic vibration apparatus, whereas spraying or fluidized bed techniques are advantageous Where the dispersion medium is a gaseous fluid.

Liquid dispersion media are suitably non-aqueous. Due to the difficulty of obtaining advantageous electrical insulating properties with aqueous media, highly insulating organic liquids are preferred. Additionally, Where the silver salt is predispersed in a hydrophilic colloid, the liquid dispersion medium is preferably one in which all dispersed components are insoluble. Such advantageous organic dlS- persion media include, for example, hydrocarbon species typically having 4-12 carbon atoms including aliphatic hydrocarbons such as isobutane, n-hexane, n-heptane, as well as other branched and straight chain hydrocarbons and cyclic hydrocarbons such as cyclohexane, etc. Additional advantageous liquid dispersion media include halogenated hydrocarbons such as carbon tetrachloride, chlorinated fiuorinated lower alkanes, isoparaffinic hydrocarbons such as Isopar G (Humble Oil and Refining Co.), and the like.

Gaseous dispersion media are widely variable and include a wide selection of materials such as air or any of the conventional aerosol propellant gases, such as nitrogen, which are substantially non-reactive with the photosensitive species or colloids described herein.

Electrostatic deposition of the subject silver salts onto a charged support from a liquid medium is promoted by the double layer charge relationship at the contact interface between the liquid medium and the dispersed solid. Particularly, with silver salt dispersions and including the silver halides, the ionic distribution can be such that a varying excess of either positive or negative ions exist at the solid surface. Such ionic distribution is related to the choice of dispersion medium. Accordingly, the relationship of the dispersed phase and the medium can affect the polarity and the magnitude of the double-layer charge. The polarity of the double layer determines whether a particle will behave like a positively or negatively charged entity under the influence of an electric field. Since the polarity is dependent upon the relationship between the dispersed phase and dispersion medium, the deposition of a silver salt can be carried out on either a positively or a negatively charged surface by utilizing favorable process conditions. Where a particular desired combination of the dispersed phase and the dispersion medium does not provide the preferred double-layer charge magnitude or polarity, a surfactant or surface active agent can be employed in conjunction with the dispersion to either impart or modify the double-layer charge that exists at the solidliquid interface. Typical surfactants include such compounds as metal salts of long chain fatty acid and cyclic organic carboxylic acids, e.g. cobalt naphthenates, aluminum stearate, etc.; long chain quaternary ammonium salts, e.g. lauryl trimethyl ammonium chloride and metal salts of long chain alkylsulfonic acids, e.g. sodium lauryl sulfonate.

When electrostatic deposition is accomplished from a gaseous fluid medium, a charge of the desired polarity can be imparted to the dispersed particles by directing them past a charged point or grid. As an example, in forming an aerosol by spray techniques, a charged source, typically a rod or pin, can be maintained near the spray head and in the concentrated (as it leaves the spray head) aerosol. The dispersed particles receive a charge of like polarity as they pass by the charged source.

According to the process of this invention, a radiationsensitive silver salt is electrostatically deposited upon an electrically insulating member which functions as the support for the resultant photographic element. These electrically insulating members, which can also be termed electrical insulators or dielectric members or supports, are advantageously formed from materials having a surface resistivity in excess of about ohms per square as measured by Van der Pauw technique, described in Phillips Research Report, 13, 1-9 (1958). Exemplary of electrically insulating support members are supports fabricated from electrically insulating resins such as cellulose triacetate, cellulose acetate butyrate, polystyrene, poly- (vinylbutyral), poly(ethylene terephthalate), polyethylene, polypropylene, etc. Alternatively, the subject supports are advantageously laminates, having at least one electrically insualting surface. In this fashion, a thin layer of an electrically insulating material, such as the described polymeric resin compositions, can be laminated to a second material, such as paper, to provide a composite support which possesses the requisite electrically insulating surface. Additionally, a support member such as paper, metals, wood, etc., can be overcoated with an electrically insulating layer to provide an advantageous composite support material. Widely varied alternative supports can be used, With the only requirement being the existence of at least one electrically insulating surface.

To obtain electrostatic coating of the silver salts described herein from a fluid medium and according to the process of this invention, an electrically insulating support such as those described elsewhere herein is first electrostatically charged such as by friction, by exposure to a corona discharge source or another like means that is effective to impart an electrostatic charge to the electrically insulating support surface. Advantageously, the surface is raised to a potential of at least about 50 volts. Conveniently used voltages conventionally range up to about 1,500 volts, but voltages in excess of 1,500 can be used for particular situations if desired. The support member, bearing an electrostatic charge on a surface thereof, is then immersed, either partially or totally, into a silver salt dispersion of this invention. Depending upon the surface charge sign of the dispersed silver salt, the support can be advantageously charged to the opposite negative or positive polarity. Since the surface charges present on the support and the dispersed silver salt grains are of opposite polarity, the silver salt particles are attracted to and deposited upon the support whereupon they from a composite, substantially binderless coating of silver halide crystals or grains. Alternatively, the silver salt crystals can be predispersed in a hydrophilic colloid such as gelatin or polyvinyl alcohol, and this resultant dispersion can then be dispersed in a liquid medium as described elsewhere herein. After the desired deposition has been obtained ,the coated support is removed from the dispersion and dried, thereby preparing a composite photographic element.

Where silver salt deposition is elfected from a gaseous fluid medium, the support, charged as described above, is exposed to an aerosol of dispersed particles which have been oppositely charged by techniques such as those previously described. Convenient aerosol formers include fluidized bed apparatus, spray apparatus, etc. The charged dispersed particles are drawn to the oppositely charged support where they adhere to form a composite, radiationsensitive photographic element. The resultant sensitive, silver salt layer can be substantially binder free or can contain a hydrophilic binder if the silver salt crystals (e.g. silver-dye complex) are predispersed in a hydrophilic colloid prior to the coating operation. In each case, whether from a liquid or gaseous medium, the dispersed silver salt can comprise either one silver salt or mixtures thereof sensitized, for example, to record light of various portions of the spectrum as is required for the preparation of full color photographic images. The uniformity of the deposit, whether it be from a liquid or gaseous medium, can be advantageously improved particularly in the case of thin coatings by having an electrode, known as a Development Electrode, separated from but in close proximity to the charged surface of the support.

The electrostatic coating process of this invention provides a number of advantages over presently employed flow or extrusion-type coating techniques for the preparation of light-sensitive, photographic layers. The amount of material deposited is primarily a function of the electrostatic charge per unit area and by the charge on the particles of the dispersed phase. As such, it is a demand situation and does not require intricate supply mechanisms such as variable pumps. Additionally, the amount of dispersion medium deposited is very small relative to the deposited radiation-sensitive silver-salt crystals, and it is not necessary to provide evaporative or other elaborate removal means for the deposited dispersion medium. Additionally, flat, curved or irregularly shaped supports can be coated with substantial uniformity by electrostatic coating means. 7 After a completely prepared photographic element is removed from contact with the dispersion and dried, it can be imagewise exposed, developed and fixed or otherwise stabilized to yield a permanent photographic silver image. Exposure is to a suitable radiation source, e.g. X-rays, visible light, ultra-violet light, etc., whereupon a metallic silver latent image is produced in the electrostatically deposited silver salt layer. Developing the latent image to a visible photographic silver image is accomplished in the case of monochromatic elements by treating the image- Wise exposed photographic element with a photographic developing composition. Such developing compositions incorporate a silver halide developing agent, typically polyhydroxy benzenes like hydroquinones, catechols and pyrogallols, as well as other polyhydroxy compounds such as ascorbic acid. Additional developing agents include aminophenols, p-phenylenediamines and 3-pyrazolidones. Exemplary of silver halide developing agents that are advantageously used herein are compounds like 2- methyl-3-chlorohydroquinone, bromohydroquinone, catechol, S-phenylcatechol, pyrogallol monomethylether(lmethoxy 2,3 dihydroxy benzene), S-methylpyrogallol monomethylether isoascorbic acid, N-methyl-p-aminophenol, dimethyl-p-phenylene diamine.

Subsequent to development, the photographic silver image is stabilized by treatment with a photographic fixing bath including a fixing agent to remove undeveloped silver salt. Conventional fixing agents or silver halide solubilizing agents include water-soluble thiosulfates, thiocyanates and mercaptans such as ammonium thiosulfate, sodium thiocyanate and the disodium salt of 2-mercapto-4-hydroxy-S-aminopyrimidine. A particularly preferred fixing agent is sodium thiosulfate. Stabilization is generally accomplished by treatment with a fixing bath that incorporates a fixing agent such as those mentioned previously. Exemplary of fixing baths is one having the formula:

Sodium thiosulfate g 240 Sodium sulfite (desiccated) g 15 Acetic acid (28% aqueous) cc 48 Boric acid (crystals) g.. 7.5 Potassium alum g 15 Water to make cc 1,000

Where mixed grain elements are prepared, such as those described herein, that are suited to the production of full color images, processing is advantageously carried out according to the technology of color reversal processing such as is described in US. 2,614,925.

A mixed grain dispersion suitable for deposition according to the present invention to prepare a full color photographic element can be made by codepositing silver halide grains that are spectrally sensitized to record particular light ranges. For example, silver chlorobromide grains that are sensitized to record red light by a photographic spectral sensitizing dye can be dispersed in a suitable medium, such as those described herein, along with silver bromide grains that are sensitized to record green light by a photographic spectral sensitizing dye. Typically, a hydrophilic colloid material is utilized as a medium in which each of the types of silver halide grains is predispersed. The codispersed silver halide grains can then be electrostatically deposited on an electrically insulating stipport according to the above-described procedures of the subject invention. Unsensitized silver halide grains treated with a yellow filter dye can then be electrostatically deposited in a second blue sensitive layer over and contiguous to the first described layer containing silver halide sensitized to record light of the red and green portions of the spectrum. Such a composite element, upon imagewise exposure and processing, produces full color photographic images.

Alternatively, a mixed grain emulsion of the type described in US. Pat. 2,388,859 can be prepared according to the electrostatic coating process of the subject invention. One photosensitive layer is electrostatically deposited, such a layer including fast silver halide grains sensitized to record the blue spectral region and slow silver halide grains sensitized to record the red and green spectral regions. The sensitivity of the grains primarily sensitive to blue light is considerably greater than the blue light sensitivity of either the red or the green sensitized silver halide grains. Over and contiguous to the photosensitive layer can be advantageously coated a yellow colored filter layer which absorbs enough blue light so that the speed of the fast blue sensitive component of the mixed grain emulsion is reduced to a value equivalent to those of the red and green speeds of the two other components. Where the blue speed diiferential between the fast and slow emulsions is, for instance, as great as fifty times, a yellow filter absorbing 98% of the overall blue light may be used. Such a filter can be one containing a dye of the pyrazolone type exemplified by tartrazine. This filter would reduce the blue speed of the red and green sensitive components of the mixed grain emulsion to approximately 2% of the speeds of the red and green thereby rendering the blue sensitivity of these grains negligible for printing purposes. The high speed blue sensitive emulsion still prints the blue record at the same speed required for printing the red and green records onto the other two components. This yellow filter layer can be, for example, flow coated from an aqueous gelatin solution subsequent to electrostatically depositing the photosensitive silver halide grains.

A mixed grain element prepared as described herein can be imagewise exposed and developed in a conventional silver halide developing composition to produce a negative silver image. Exemplary of such a developer is one having the formula:

N-methyl-p-aminophenol sulfate 6 Hydroquinone 10 Sodium sulfite 50 Sodium carbonate 30 Potassium bromide l-phenyl-S-mercapto tetrazole 0.01

Water to 1 liter.

The element after negative development has a negative silver image produced by the blue exposure, a negative silver image produced by the green exposure and a negative silver image produced by the red exposure. In each layer, there remain unexposed and undeveloped grains having the three recited light sensitivities.

After washing, the film can then be light-exposed through the base through a red filter transmitting light of longer wavelength than about 640 millimicrons, to expose the residual red sensitive grains and can then be developed in a color-forming developer which can have the following composition:

p-Aminodiethylaniline HCl 2 Sodium sulfite 6 Sodium carbonate 50 Potassium bromide 5 Potassium thiocyanate 1 Water to 950 cc.

o-Hydroxydiphenol 3 Sodium hydroxide 5 Water to 50 cc.

(For use, Solution B is added to Solution A.)

The film can then be exposed from the support side to green light to expose the residual green sensitive grains and the film can then be developed in a developer which may have the following composition:

p-Aminodiethylaniline HCl 1 /2 Sodium sulfite 1 Potassium bromide 2 Water to 950 cc.

1-(p-nitrophenyl)-3-methyl-5-pyrazolone 3 Water to 50 cc.

(For use, Solution B is added to Solution A.)

The film can then be exposed to white light to expose the residual blue sensitive silver halide grains and developed in a developer which may have the following composition:

G. p-Aminodiethylaniline HCl 1 Sodium sulfite 1 Potassium thiocyanate 1 Water to 990 cc.

l Acetoacetanilide 10 Sodium hydroxide 1 Water to 10 cc.

(For use, Solution B is added to Solution A.)

After development in the yellow color-forming developer, the film is washed and bleached in a solution having a formulation such as the following composition:

Potassium ferricyanide g 50 Ammonium hydroxide (28% solution) cc 2 /2 Potassium bromide g 5 Water to 1 liter.

The bleaching converts silver metal to a soluble silver salt. The film is then typically fixed for about 5 minutes in hypo, washed for several minutes to remove residual silver salts including silver halide and the soluble silver salts resulting from the ferricyanide bleach, and dried in warm air. After processing and drying the mixed grain photographic element, the electrically insulating support material carries a visible photographic dye image that is stable and can be viewed either by reflected light or by transmitted light, such as by projection, when the support material is itself transparent, as in the case of glass, poly (ethylene terephthalate), polystyrene, cellulose acetate and other conventional polymeric resins used as photographic film base materials, and has a transparent insulating surface.

Where the radiation-sensitive layer has, as the sensitive component, silver-dye complex crystals or mixtures thereof, imagewise exposure to a suitable source of radiation is followed by physical development to prepare a photographic image. Exposure of the sensitive silver-dye complex produces physical development nuclei in exposed areas.

Physical development is conveniently accomplished, for example, by contacting the exposed silver-dye complex layer with a conventional photographic emulsion in the presence of a silver halide developer containing a silver halide solvent and then after a short time removing the silver halide emulsion in the light to reveal a silver deposit in the areas of exposure in the exposed silver-dye complex layer. A positive image will then be obtained in the silver halide emulsion when it is subsequently exposed, which can be suitably fixed if desired. It may not be necessary to fix the positive image, depending upon the time of transfer and the amount of silver halide present in the silver donor layer. As an alternative developing system, a physical developer solution may be used rather than the diffusion transfer method of development.

Processing of the imagewise-exposed photographic elements utilizing a silver-dye complex is conveniently accomplished in a physical developer solution containing heavy metal ions in salt form and a reducing agent for the metal ions, and when the metal ion salt is substantially insoluble in water, a solvent for the metal ion salt, such as water-soluble thiosulfates, thiocyanates, etc., to produce a visible photographic image corresponding to the exposed areas having development centers. Typical physical developer solutions are well known (see Hornsky, Basic Photographic Chemistry (1956), 66, and Mees and James, ed., The Theory of the Photographic Process, 3rd ed. (1966), 329-331) and contain the metallic ions such as silver, copper, iron, nickel and cobalt necessary to form a visible image at and in the vicinity of nucleating centers, the microscopic metal deposits created during the above described first step. Typical reducing agents used in the physical developer include, for example, polyhydroxy-substituted aryl compounds such as hydroquinones, catechols and pyrogallols; ascorbic acid derivatives; aminophenols; p-phenylenediamines, and the like developing agents used in the photographic art. Particular examples of reducing agents for physical developer solutions are 2-methyl-3-chlorohydroquinone, bromohydroquinone, catechol, S-phenylcatechol, pyrogallol monomethyl ether (1-methoxy-2,3-dihydroxybenzene) and 5- methylpyrogallol monomethyl ether, isoascorbic acid, N- methyl-p-aminophenol, dimethyl-p-phenylenediamine, 4- amino-N,N-di (n-propyl)aniline and 6-amino-1-ethyl- 1,2,3,4-tetrahydroquinoline. The completely developed element carries a visible image, typically metallic silver, corresponding to the exposed areas. As such, it is a negative reproduction of the original pattern, and when a negative serves as the original pattern, positive copies are obtained.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 One gram of dry silver bromide crystals from a conventional photographic silver bromide-gelatin dispersion which is treated according to the enzyme hydrolysis technique of Weiss, Ericson and Herz, Journal of Colloid and Interface Science, 23, p. 277 (1967), to remove substantially all of the gelatin is crushed in a stainless steel beaker under spectrograde n-hexane using a glass rod. Additional hexane is added to bring the total volume to 500 ml., and the mixture is agitated by exposure to ultrasonic vibrations for about two minutes. The mixture is allowed to settle for 30 seconds and the suspension is decanted into a clean dry stainless steel beaker leaving the unsuspended solids behind. All operations involving the silver bromide are performed under red safelights; 2 x

cm. pieces of polyethylene-coated paper serve as the support material. Five 17 x 20 mm. areas on each support strip are electrostatically charged by exposure to a corona through a screen grid in such a way as to charge each of the areas to a diflerent potential from to 1000 volts. All charged areas on any one strip have the same polarity which can be either positive or negative. A positively charged support strip is bathed for 30 seconds in the silver bromide-hexane dispersion, removed, and allowed to dry, thereby preparing a photographic element having a substantially binderless layer of crystalline silver bromide. Visual examination indicates that silver bromide is deposited predominantly in the charged areas. The treated support strip is then exposed for one second through a neutral density step tablet to the ambient room light provided by watt tungsten lamps positioned at a distance of about five feet from the exposure plane. The exposed element is then developed for 15 seconds in a photographic developer solution having the formula:

G. Sodium carbonate 26.0 Sodium sulfite 26.0 Elon (N-methyl-p-aminophenol sulfate) .67 Hydroquinone 2.5 KBr .67 Gelatin 1.67

Water to 1 liter.

Adjust pH to 10.3 bathed about 15 seconds in a 3% acetic acid solution, then fixed for 5 minutes in a fixing bath having the formula:

Water cc 500 Sodium thiosulfate (hypo) gm 240 Sodium sulfite, desiccated gm 10 Sodium bisulfite gm 25 Cold Water to cc 1000 and rinsed under running water for 10 seconds. Images of the step tablet are produced on the areas that have been charged by the corona. The thickness of the silver bromide, as determined by the reflectance density of the developed silver image, is directly related to the electrostatic potential of the coated area.

EXAMPLE 2 Five ml. of 6% cobalt naphthenate is added to the AgBr-hexane dispersion used in Example 1, and the dispersion is agitated with ultrasonic vibration for about 1 minutes as in Example 1. Both a negatively and a positive- 1y charged support strip, as in Example 1, are bathed simultaneously in the dispersion for 30 seconds and dried. Visual examination indicates that the negatively charged support receives a deposit only on the charged areas whereas the silver bromide is deposited immediately adjacent to the charged areas on the positively charged strip. Both samples are exposed and photographically processed as 1n Example '1. Images of the step tablet are produced on the charged areas of the negative strip and immediately surrounding the charged areas on the positive strip. The thickness of the silver bromide, as determined by the reflectance density of the developed silver image, is directly related to the electrostatic potential of the charged area that the silver was either on or around.

EXAMPLE 3 Electrostatic coating of silver halide-gelatin emulsion: 15 grams of a gelatino, silver bromoiodide emulsion (.0053 mole) is added to 300 ml. of cyclohexane containing 0.15 gram sorbitan monooleate. This mixture is then blended by stirring for 8 minutes. Further dispersion of the emulsion is carried out by treatment with ultrasonic vibration, as in Example 1, for a period of one minute. A 12 x 35 mm. strip of polyethylene coated paper support which is given a negative surface charge of 1000 volts is then coated with the silver halide-gelatin emulsion by immers- 1 1 ing the support strip into the dispersion for seconds, withdrawing and drying. The resultant photographic element is then exposed to a tungsten light source on a .3 log E wedge spectrograph, developed 30 seconds in a photographic developer solution, fixed, washed and dried, all processing as in Example 1. A photographic silver image of the spectrograph is produced in the photographic layer.

EXAMPLE 4 A gelatino, silver bromoiodide emulsion as described in Example 3 but also including the spectral sensitizing dye 3,3-diethyl-9-methylthiacarbocyanine bromide in an amount of 100 mg. per mole of silver based on the silver halide. After coating, exposure and development as in Example 3, the photographic element exhibits an image of the spectrograph with increased sensitization being achieved over a broad band of from 400 to 600 nm.

EXAMPLE 5 Electrostatic deposition from an aerosol dispersion: grams of a fine grain gelatin bromoiodide emulsion containing 1 mole silver, 25 grams gelatin and 25 grams of copoly 3-thiapentylacrylate-3-acryloxypropanel-sulfonic acid, sodium salt per 1.09 kilograms is added to 300 ml. of a pure grade cyclohexane containing 0.30 gram sorbitan monooleate, and the mixture is blended with stirring for 8 minutes. A development electrode sandwich is then prepared from two stainless steel plates x 2%" x 6"), two brass shims (0.10" x 0.5" x 6") and a strip of polyethylene-coated photographic paper having a region negatively charged to a potential of 1000 volts. By making up a sandwich of steel-brass-paper-brass-steel, the charged paper is arranged so that a development electrode is 0.01" from both faces of the paper. One open end of this sandwich is fixed to the hose of an aspirator pump and the other is situated 3" from a 4 x 4" corona generator operating at 11 kv. and 0.05 milliamp. About 50 ml. of the prepared emulsion-cyclohexane dispersion is then sprayed through the corona screen toward the open end of the sandwich. The paper is then removed from the sandwich and allowed to dry. Half of the strip is exposed to a tungsten light source, as in Example 1, developed, fixed, washed and dried, all as in Example '1. A photographic silver image appears where the element is exposed, and it is evident that the silver halide is deposited on the negatively charged area.

EXAMPLE 6 Two dispersions of silver bromoiodide emulsions are prepared as in Example 3 except that each portion of silver bromoiodide is treated with a different spectral sensitizing dye, the first being sensitized to record red light with 8- phenyl-3,4,3',4-dibenzothiacarbocyanine bromide and the second being sensitized to record green light with 3,4- beuzothia-2-cyanine bromide. The solids are separated, washed and codispersed in equal quantities (5 g. each) in cyclohexane containing sorbitan monooleate as in Example 3 and according to the stirring and ultrasonic dispersing technique of Example 3. Electrostatic coating of this mixed grain dispersion is accomplished as in Example 3. After drying, portions of the composite photographic element are first exposed on a wedge spectrograph through red and green filters respectively and then developed and fixed according to the procedure of Example 1. The resultant negative silver images indicate the desirable red and green light sensitivity present in the coated layer.

With the omission of the above fixing step and subsequent exposures to red and green radiation and treatment in the proper sequence with color-forming developers, as is described hereinabove, the preparation of a two-color (red-geen) positive image can be accomplished. Altenatively, color-forming couplers can be incorporated into the dispersions and processing with conventional developers can be used to prepare colored, postive photographic dye images.

12 The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim: 1. An electrostatic coating process for preparing a photographic element comprising a support having an electrically insulating surface of a surface resistivity in excess of about 10 ohms per square on which is coated a radiation-sensitive layer comprising preformed silver salt crystals, said process comprising:

dispersing the preformed radiation-sensitive silver salt crystals in a dielectric liquid medium to form on the preformed crystal surfaces a charge of a first polarity,

preliminarily forming on the insulating surface of the support an electrostatic charge of a second polarity opposite that of the first polarity and differing in potential by at least 50 volts,

bringing the electrostatically charged surface of the support into contact with the dielectric liquid medium having dispersed therein the surface charged preformed crystals, and

preferentially and electrostatically attracting and depositing the preformed charged crystals from the dielectric liquid medium onto the charged surface of the support.

2. An electrostatic, silver-salt coating process, as described in claim 1, wherein the silver salt is predispersed in a hydrophilic colloid.

3. An electrostatic, silver-salt coating process, as described in claim 1, wherein the silver salt is a silver halide salt selected from the group consisting of silver bromide, silver chloride, silver iodide and co-crystals and mixtures thereof.

4. An electrostatic silver-salt coating process, as described in claim 2, wherein the silver salt is a silver-dye complex.

5. An electrostatic, silver-salt coating process, as described in claim 1, wherein the electrically insulating support comprises an electrically insulating resin material.

6. An electrostatic, silver-salt coating process as described in claim 1, wherein the electrically insulating support is selected from the group consisting of polyethylene, polyethylene-coated paper, polystyrene and poly (ethylene terephthalate) 7. An electrostatic, silver-salt coating process, as described in claim 1 in which said dielectric liquid medium is an aliphatic hydrocarbon or halogenated hydrocarbon liquid dispersion medium.

8. An electrostatic, silver-salt coating process, as described in claim 1 in which said dielectric liquid medium is n-hexane.

9. An electrostatic coating process for preparing a photographic element comprising a support having an electrically insulating surface of a surface resistivity in excess of about 10 ohms per square on which is coated a radiation-sensitive layer comprising preformed silver salt crystals, said process comprising:

dispersing the preformed radiation-sensitive silver salt crystals in a gaseous dielectric fluid medium while forming on the preformed crystal surfaces a charge of a first polarity,

preliminarily forming on the insulating surface of the support an electrostatic charge of a second polarity opposite that of the first polarity and differing in potential by at least 5 0 volts,

bringing the electrostatically charged surface of the support into contact with the dielectric gaseous medium having dispersed therein the surface charged preformed crystals, and

preferentially and electrostatically attracting and depositing the prcformed charged crystals from the dielectric gaseous medium onto the charged surface of the support.

10. An electrostatic, silver salt coating process as described in claim 9 wherein the silver salt crystals are dispersed by spraying.

11. An electrostatic, silver salt coating process as described in claim 9 wherein the silver salt crystals are dispersed by fluidized bed dispersion means.

12. An electrostatic, silver salt coating process as described in claim 9 wherein the silver salt crystals are silver halide crystals selected from the group consisting of silver bromide, silver chloride, silver iodide and cocrystals and mixtures thereof.

References Cited UNITED STATES PATENTS 3,335,026 8/1967 De Geest et a1 l1793.4 3,113,037 12/1963 Watanabe 117-93.4

1 4 3,219,445 11/1965 LuValle et a1 96--94 BF 2,952,559 9/1960 Nadeau 1l7-93.4 X

2,695,002 11/1954 Miller 11793.4

FOREIGN PATENTS 746,549 11/1966 Canada 117-34 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. XR.

96-94 BF; 117-17, 37 LE, 47 A, 93.4 NC; 204181 

